Color diffusion transfer film units are known to the art which are designed to be processed outside of the camera. In order to avoid fogging the exposed silver halide emulsion layer, various means have been proposed to prevent such an occurrance. For example, the photosensitive system may be sandwiched between opaque supports; or between an integral opaque-reflective layer and an opaque support. U.S. Pat. No. 3,647,437, issued Mar. 7, 1972, to Edwin H. Land, discloses, in one embodiment (FIG. 18), a film unit which comprises a transparent support carrying a layer of polymeric acid, a spacer layer, an image-receiving layer, an opaque layer, which includes a white pigment to provide a white background against which the image may be viewed, a dye developer layer, a silver halide emulsion layer, and a colorless layer containing an alkali-activated optical filter agent. The processing fluid is applied by rupturing a pod and distributing the processing fluid between a cover sheet and the colorless layer. The cover sheet may also be opaque. It is also stated that the colorless layer may also contain suitable opacifying agents such as carbon black, titanium dioxide etc., where opacifying capability additional to that provided by the colorless layer is required.
U.S. Pat. No. 4,407,929, issued Oct. 4, 1983 to Immo Boie, et al. is directed to a diffusion transfer film unit which includes a transparent support, at least one photosensitive silver halide emulsion layer having non-diffusible color providing material associated therewith, an opaque light-reflecting layer and an image-receiving layer. Boie further states that the transparent support may carry a layer which would improve flatness and improve color definition. Thus, Boie is providing a material which functions as a mask. For example, Boie proposes a material on the transparent support which will produce a white haze when contacted with the developer bath.
U.S. Pat. No. 3,647,435, issued Mar. 7, 1972 to Edwin H. Land, is referred to in Boie under the corresponding number DE-OS 2,127,924. Land discloses diffusion transfer film units which contain precursors adapted to form reflecting agents to provide a mask for the opacifying agent.